Conductive inks or pastes are used to form metal contacts, such as silver gridlines and bus bars, on the surface of substrates such as silicon. Such substrates can be used in solar cells or photovoltaic cells that convert solar energy to electrical energy when photons from sunlight excite electrons in semiconductors from the valance band to the conduction band. The electrons which flow in the conduction band are collected by the metal contacts. Crystalline silicon solar cells in today's industry are typically coated with an anti-reflection coating to promote light adsorption, which increases cell efficiency. However the anti-reflection coating also works as an insulator by preventing electrons from transferring from the substrate to the metal contacts. The anti-reflection coatings often comprise silicon nitride, titanium oxide or silicon oxide.
Conductive inks typically include a glass frit, metal particles or conductive species and an organic medium. The metal particles, usually silver particles, provide conductive properties and function as current collectors after formation of the metal contacts. To form the metal contacts, conductive inks are printed onto the substrate. The substrate is then fired at a temperature in the range of about 650° C. to about 950° C. In most instances, a sintering aid is needed because the firing temperature used is lower than the eutectic point of silver and silicon, and the silver melting point. In addition, the conductive ink should penetrate the anti-reflection coating to form metal contacts having ohmic contact with the substrate.
Conventional conductive pastes incorporate glass frits to aid with sintering the metal particles to a substrate and to promote adhesion and ohmic contact between the formed metal contact and the substrate. Depending on the formulation, glass frits can liquefy upon firing at a temperature between about 300° C. and 600° C. When the glass frit liquefies, it tends to flow toward the interface between the metal particles or silver particles and the anti-reflection coating disposed on the substrate. The melted glass dissolves the anti-reflection coating materials as well as some of the silver and substrate. Once the temperature decreases, the molten silver and the melted or dissolved substrate recrystallize through the liquid phase. As a result, some of the silver crystallites are able to penetrate the antireflective layer and form ohmic contact with the substrate. This process is referred to as “fire-through” and facilitates a low contact resistance formation and a stronger bond between silver and the substrate.
As will be discussed herein, glass frits are not believed to be ideal materials for use in the fire-through process and, therefore, a need for substitute materials exists. The use of metallo-organics in conductive inks, which do not include glass frit, is discussed in the article entitled Silver Thick Film Metallization for Photovoltaics Fired at 300° C.,” by C. J. Sabo, et al. (herein referred to as the “Sabo Article”). The Sabo Article specifically discusses using a silver metallo-organic component, such as silver neodecanoate, in a conductive paste or ink to be screen printed onto a silicon wafer to form gridlines. The Sabo Article abstract states that printed inks were applied to silicon wafers or solar cells, dried for 30 minutes at 65° C. and fired at a maximum temperature of 300° C. for 70 minutes.
Accordingly, there continues to be a need for other glass frit substitutes for use in conductive inks which aid in sintering, reduce resistivity in photovoltaic applications, and also have the capability of promoting adhesion and ohmic contact through anti-reflection coatings.